1. Field of the Invention
The present invention relates generally to the control of loads supplied by an A.C. voltage and, more specifically, to the field of power dimmers of a resistive or inductive load.
The present invention more specifically relates to the forming of a power dimmer based on switches bidirectional for the voltage and unidirectional for the current which are assembled in anti-parallel. A recurrent problem then concerns the supply of the respective control circuits of the two switches.
2. Discussion of the Related Art
FIG. 1 schematically shows a conventional example of a circuit 1 for controlling a load Q supplied by an A.C. voltage Vac. Two switches K1 and K2 are assembled in anti-parallel between two power terminals 2 and 3 of the dimmer, connected in series with load Q between two terminals P and N of application of A.C. voltage Vac. Each switch K1, K2 has its control terminal connected to the output of a respective circuit 4 (DRIV1) and 5 (DRIV2), generating an appropriate control signal based on control reference values received from a common control circuit (CTRL) 6. Circuit 6 receives a power reference intended for the load, for example, by means of a potentiometer 7 adjusting a reference voltage exploited by circuit 6.
FIGS. 2A to 2D illustrate the operation of the power dimmer of FIG. 1, the principle of which is a phase angle control. FIGS. 2A to 2D respectively show examples of shapes of voltage Vac, of current IQ in load Q, and of the on periods of switches K1 and K2.
It is assumed that load Q is an inductive load, whereby its current is phase-shifted with respect to the A.C. voltage.
The phase angle control consists of controlling the turn-on time of one of switches K1 or K2 according to the considered halfwave, starting from the beginning of the A.C. supply halfwave. Indeed, for each halfwave, respective turn-on times t1 and t2, of switches K1 or K2 cause the occurrence of a current in the load to be controlled. The turning-off of the on switch (time t1′, t2′, respectively) is controlled by a detection of the disappearing of current IQ in the load, which reproduces a triac-type operation. Since an inductive load is assumed, times t1′ and t2′, respectively, of switch turn-off, and thus of the disappearing of the current in the load, are shifted with respect to the zero-crossing times of A.C. voltage Vac.
The constraints to be fulfilled by a control circuit such as illustrated in FIG. 1 are to provide the reference voltages to the control electrodes of the semiconductor switches, to transfer the information from control circuit 6 to the circuit (here, 5) which does not have the same voltage reference, and to detect the zero crossing of the current in the conductive switch.
In conventional circuits of this type, the fulfilling of these constraints imposes use of two independent external power supplies to provide voltages Vcc1 and Vcc2, and thus generally of a transformer. Further, the information transfer for the stage (for example, 5) which does not have the same reference voltage as control circuit 6 requires an isolation barrier 8 (IB) of the optocoupler, pulse transformer, or level shifter type.
Further, to enable turning off at the current zeroes in the load and thus avoiding overvoltages adversely affecting the turning-off, each circuit 4, 5 typically has an input terminal connected to a power terminal of the dimmer (respectively, terminal 2 for switch K1 and terminal 3 for switch K2). In practice, this generally leads to providing a sense resistor (not shown) in series with each of the switches to measure the voltage zeroes that correspond to the current zeroes. Such a resistor generates undesirable losses when the switch is in the on state.